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US prosecutors have decided to drop criminal charges against two former JP Morgan Chase & Co derivatives traders implicated in the “London Whale” trading scandal that caused $6.2bn (£4.7bn) of losses in 2012.

In seeking the dismissal of charges against Javier Martin-Artajo and Julien Grout, the Department of Justice said it “no longer believes that it can rely on the testimony” of Bruno Iksil, the trader dubbed the London Whale, based on recent statements and writings he made that hurt the case.

Prosecutors also said efforts to extradite Martin-Artajo and Grout, respectively citizens of Spain and France, to face the charges have been “unsuccessful or deemed futile”.

The acting US attorney in Manhattan, Joon Kim, asked a federal judge for permission to drop charges that included securities fraud, wire fraud and falsifying records. Martin-Artajo and Grout were indicted in September 2013.

US prosecutors charge two JP Morgan traders over 'London Whale' incident

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“After four long years of protracted litigation, we are very pleased that the government has decided to do the right thing, and dismiss the criminal case,” said Grout’s lawyer, Edward Little.

Lawyers for Martin-Artajo did not immediately respond to requests for comment.

Martin-Artajo and Grout had been accused of hiding hundreds of millions of dollars of losses within JP Morgan’s chief investment office (CIO) in London by marking positions in a credit derivatives portfolio at inflated prices. The losses were part of the $6.2bn loss centred on Iksil, who Martin-Artajo supervised and for whom Grout worked.

Prosecutors said Martin-Artajo and Grout acted in part to enhance their prospects for promotions and bonuses.

The scandal briefly hurt the reputation of JP Morgan chief executive Jamie Dimon, who initially called it a “tempest in a teapot”. JP Morgan ultimately paid more than $1bn and admitted wrongdoing to settle related US and British investigations.

Iksil has chafed at the “London Whale” moniker and being portrayed as solely at fault for the losses. In a February 2016 letter released to the media, the French national said he had been “instructed repeatedly” by senior management in the CIO to execute the trading strategy that caused the losses.

Martin-Artajo and Grout still face US Securities and Exchange Commission civil charges over the scandal.

Iksil’s lawyer and JP Morgan did not immediately respond to requests for comment.

Question: What could have prevented the London Whale incident from happening?